Isoconjugate hydrocarbon model

Both models can be extended. The localized orbital model is improved by allowing for the interaction between the locally excited benzene states and the CT state, and the isoconjugate hydrocarbon model is improved by taking into account the electronegativity change upon the replacement of the C atom in the benzyl anion by an N atom in aniline. Good agreement with experiment is obtained in both cases. 

A rigorous theoretical treatment of the non-alternant and heterocyclic indolizine is extremely difficult and, even for the related isoconjugate hydrocarbon, far from conclusive. Many questions, however, in which experimentalists are interested may be answered in a satisfactory way on the basis of a perturbational treatment. This approach has been used for a discussion of the electronic spectra of indolizine and some azaindolizines (63JCS3999). Following first-order PMO theory the 7r-stabilization which follows from aza substitution at the different positions of the model molecule depends on the ir-electron density qt as well as the change in electronegativity Sat (B-75MI30801). The perturbations caused by aza substitution of the indenyl anion are depicted in Scheme 1. 

It may be of interest that isoconjugate benzenoid hydrocarbons necessarily have the same RE in Herndon s resonance theory VB model as in the conjugated circuits model, but generally they will have different RE in the HMO model and the TRE model, mentioned earlier. This clearly points to not only numerical but also conceptual differences between different graph theoretical models those based on the adjacency matrix (HMO) and that based on Kekule valence structures and conjugated circuits (VB). 

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